Remove LetterboxEdu in DesktopWindowing

The LetterboxEducation will be removed when already displayed and
the user enters DesktopWindow.

Flag: com.android.window.flags.skip_compat_ui_education_in_desktop_mode
Fix: 416501619
Test: atest WMShellUnitTests:CompatUIControllerTest

Change-Id: I4ebcbd9a89e9687a69bf4b9dfb78b1cbe7baabee